IgG Fc-binding protein positively regulates the assembly of pore-forming protein complex ßγ-CAT evolved to drive cell vesicular delivery and transport.

Cell membranes form barriers for molecule exchange between the cytosol and the extracellular environments. ßγ-CAT, a complex of pore-forming protein BmALP1 (two ßγ-crystallin domains with an aerolysin pore-forming domain) and the trefoil factor BmTFF3, has been identified in toad Bombina maxima. It plays pivotal roles, via inducing channel formation in various intracellular or extracellular vesicles, as well as in nutrient acquisition, maintaining water balance, and antigen presentation. Thus, such a protein machine should be tightly regulated. Indeed, BmALP3 (a paralog of BmALP1) oxidizes BmALP1 to form a water-soluble polymer, leading to dissociation of the ßγ-CAT complex and loss of biological activity. Here, we found that the B. maxima IgG Fc-binding protein (FCGBP), a well-conserved vertebrate mucin-like protein with unknown functions, acted as a positive regulator for ßγ-CAT complex assembly. The interactions among FCGBP, BmALP1, and BmTFF3 were revealed by co-immunoprecipitation assays. Interestingly, FCGBP reversed the inhibitory effect of BmALP3 on the ßγ-CAT complex. Furthermore, FCGBP reduced BmALP1 polymers and facilitated the assembly of ßγ-CAT with the biological pore-forming activity in the presence of BmTFF3. Our findings define the role of FCGBP in mediating the assembly of a pore-forming protein machine evolved to drive cell vesicular delivery and transport.

Pore-forming protein ßγ-CAT promptly responses to fasting with capacity to deliver macromolecular nutrients.

During animal fasting, the nutrient supply and metabolism switch from carbohydrates to a new reliance on the catabolism of energy-dense lipid stores. Assembled under tight regulation, ßγ-CAT (a complex of non-lens ßγ-crystallin and trefoil factor) is a pore-forming protein and trefoil factor complex identified in toad Bombina maxima. Here, we determined that this protein complex is a constitutive component in toad blood, that actively responds to the animal fasting. The protein complex was able to promote cellular albumin and albumin-bound fatty acid (FA) uptake in a variety of epithelial and endothelial cells, and the effects were attenuated by a macropinocytosis inhibitor. Endothelial cell-derived exosomes containing largely enriched albumin and FAs, called nutrisomes, were released in the presence of ßγ-CAT. These specific nutrient vesicles were readily taken up by starved myoblast cells to support their survival. The results uncovered that pore-forming protein ßγ-CAT is a fasting responsive element able to drive cell vesicular import and export of macromolecular nutrients.

A cellular endolysosome-modulating pore-forming protein from a toad is negatively regulated by its paralog under oxidizing conditions.

Endolysosomes are key players in cell physiology, including molecular exchange, immunity, and environmental adaptation. They are the molecular targets of some pore-forming aerolysin-like proteins (ALPs) that are widely distributed in animals and plants and are functionally related to bacterial toxin aerolysins. ßγ-CAT is a complex of an ALP (BmALP1) and a trefoil factor (BmTFF3) in the firebelly toad (Bombina maxima). It is the first example of a secreted endogenous pore-forming protein that modulates the biochemical properties of endolysosomes by inducing pore formation in these intracellular vesicles. Here, using a large array of biochemical and cell biology methods, we report the identification of BmALP3, a paralog of BmALP1 that lacks membrane pore-forming capacity. We noted that both BmALP3 and BmALP1 contain a conserved cysteine in their C-terminal regions. BmALP3 was readily oxidized to a disulfide bond-linked homodimer, and this homodimer then oxidized BmALP1 via disulfide bond exchange, resulting in the dissociation of ßγ-CAT subunits and the elimination of biological activity. Consistent with its behavior in vitro, BmALP3 sensed environmental oxygen tension in vivo, leading to modulation of ßγ-CAT activity. Interestingly, we found that this C-terminal cysteine site is well conserved in numerous vertebrate ALPs. These findings uncover the existence of a regulatory ALP (BmALP3) that modulates the activity of an active ALP (BmALP1) in a redox-dependent manner, a property that differs from those of bacterial toxin aerolysins.

Amphibian pore-forming protein ßγ-CAT drives extracellular nutrient scavenging under cell nutrient deficiency.

Nutrient acquisition is essential for animal cells. ßγ-CAT is a pore-forming protein (PFP) and trefoil factor complex assembled under tight regulation identified in toad Bombina maxima. Here, we reported that B. maxima cells secreted ßγ-CAT under glucose, glutamine, and pyruvate deficiency to scavenge extracellular proteins for their nutrient supply and survival. AMPK signaling positively regulated the expression and secretion of ßγ-CAT. The PFP complex selectively bound extracellular proteins and promoted proteins uptake through endolysosomal pathways. Elevated intracellular amino acids, enhanced ATP production, and eventually prolonged cell survival were observed in the presence of ßγ-CAT and extracellular proteins. Liposome assays indicated that high concentration of ATP negatively regulated the opening of ßγ-CAT channels. Collectively, these results uncovered that ßγ-CAT is an essential element in cell nutrient scavenging under cell nutrient deficiency by driving vesicular uptake of extracellular proteins, providing a new paradigm for PFPs in cell nutrient acquisition and metabolic flexibility.